To identify and isolate mouse myeloma cells which have undergone mutations in immunoglobulin production and then to characterize the biochemical defects in these mutant cell lines. A technique has been developed for detecting cells which have undergone rare mutations in immunoglobulin production. These mutant cells are induced with mutagenizing agents and appear to be true mutants in that they are making defective gene products. Using radioactive precursors, pulse-chase experiments, and SDS acrylamide gels, blocks in synthesis, assembly, and secretion will be detected. The biological function of mutant molecules will also be examined.